The present invention relates to acid treatments for ceramic articles and particularly involves the treatment of lithium-containing beta-quartz ceramics with strong acids to non-destructively remove lithium therefrom by an ion exchange process.
Beta-quartz (high quartz) is a hexogonal crystalline form of SiO.sub.2, stable from 573.degree.-870.degree. C., which exhibits a slightly negative coefficient of thermal expansion. This form of silica is known to enter into solid solution with crystal species of analogous structure such as beta-eucryptite (Li.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2), with the resulting quartz derivatives being denominated beta-quartz solid or "stuffed derivatives" of beta quartz. Some of these derivatives are described by M. J. Buerger in "The Stuffed Derivatives of the Silica Structures", Am. Mineral, 39, 600-614 (1954).
Ceramic articles, particularly glass-ceramic articles, are known wherein hexagonal beta-quartz solid solution comprises a principal crystal phase. U.S. Pat. No. 3,252,811 to Beall, for example, describes glass compositions suitable for the manufacture of transparent glass-ceramics wherein lithium, magnesium, or zinc-containing beta-quartz solid solutions constitute the crystal phases formed on heat treatment, making up more than about 50% by volume of the glass-ceramic articles. Similarly, U.S. Pat. No. 3,681,097 to Beall and Martin discloses glass-ceramics which comprise a zinc-stuffed beta-quartz crystal phase, such phase constituting more than about 50% by weight of the glass-ceramics.
Pure beta-quartz exhibits a displacive transformation at about 573.degree. C., transforming to the higher expansion alpha-quartz (low quartz) form at temperatures below this point. Certain beta-quartz solid solutions of lithium aluminosilicate composition have recently been found to exhibit similar displacive transformations on cooling.
Ceramic articles comprising a lithium-containing beta-quartz solid solution crystal phase can be formed from lithium aluminosilicate batch compositions having a silica to alumina mole ratio (SiO.sub.2 :Al.sub.2 O.sub.3) within the range of about 3.5 to 10. The degree of lithium beta-quartz crystallinity in the article is enhanced by maintaining an alumina:lithium oxide ratio (Li.sub.2 O:Al.sub.2 O.sub.3) near 1:1, since solid solution is deemed to occur by substitution of Li.sup.+ + Al.sup.+3 units for Si.sup.+4 units in the beta-quartz structure.
As is well known, lithium aluminosilicate ceramic batch compositions such as above described are also useful in the production of ceramic articles wherein beta-spodumene solid solution comprises a principal crystal phase. Beta-spodumene solid solutions are quite distinct from beta-quartz solid solutions, exhibiting a tetragonal crystal structure, a very low coefficient of expansion, and no displacive transformation on cooling. However, control over the crystal phase produced in lithium aluminosilicate compositions can be exercised by selecting the conditions of crystallization, most particularly the heat treatment temperature, and the nucleating agents, if any, utilized to promote crystal development, as is also well known.
The extraction of lithium from mineral beta-spodumene by a hydrogen-for-lithium ion exchange has been known since at least 1950, being described in U.S. Pat. No. 2,516,109 to Ellestad and Leute. More recently, Grossman and Rittler disclosed in U.S. Pat. No. 3,834,981 that lithium may be extracted from unitary ceramic materials comprising beta-spodumene solid solution crystals by the same hydrogen-for-lithium ion exchange. The extraction process is non-destructive, and a ceramic product comprising a new phase, termed "aluminous keatite", is produced by firing the ion exchanged material.